The Diversity of Long-Term Persistent WT1-Specific Cytotoxic T Lymphocytes after Wilms' Tumor 1 Peptide Vaccination

[Introduction] Remarkable advances have been made in cancer immunotherapy, including the development of peptide-based cancer vaccines. Wilms' tumor 1 (WT1) is one of the cancer-testis antigens, and the WT1 gene is overexpressed in hematologic malignancies. Several clinical trials of WT1 peptide vaccines showed promising efficacy and high safety of the vaccines for hematologic malignancies. In these trials, immunological responses were assessed within 2 years after vaccination, and the transient WT1-specific immune response observed in many cases early after vaccination was confirmed. However, the long-term durability of the response of WT1-specific CD8+ cytotoxic T lymphocytes (CTLs) after peptide vaccine therapy and the T-cell receptor (TCR) diversity in those CTLs has not been clarified. More than 10 years ago, a patient with chronic myeloid leukemia (CML) received WT1 peptide vaccination after the failure of tyrosine kinase inhibitor therapy. After vaccination, WT1-specific CD8+ CTLs were observed. We continued the immunological assessment of the patient for more than 10 years after the WT1 peptide vaccination. Herein, we report our findings from the long-term monitoring of WT1-specific CTLs in the patient with CML and describe the results of our detailed analysis, including the functionality and clonality of the CTLs.